How do you measure uptime?

Uptime is the percentage of time your site or service is available during a given period. The formula is simple:

uptime % = (available time / total time) × 100

So if your site was up for 43,200 out of 43,200 minutes in a 30-day month, that's 100%. If it was down for 44 minutes, you're at 99.9%.

How monitoring services actually measure it

Good uptime monitoring tools do it by running a consistent check (typically HTTP, Ping, or TCP) at a fixed interval (every minute for most services), and recording whether each check succeeded or failed. When enough consecutive checks fail to count as an outage, the monitor logs the start and end time and deducts that window from the uptime total.

To keep measurements honest, a solid monitor will:

• Run checks frequently (every minute is standard)
• Verify failures from a second location before recording an outage
• Store the full history, so you can calculate uptime for any time window after the fact
• Let you exclude planned maintenance windows so scheduled work doesn't skew the number

Oh Dear does all of that automatically. You can see any monitor's uptime at a glance and export a full incident history for SLA reporting.